In the technique of packaging the use of packing material comprising outer layers of thermoplastics has been known for a long time, where these outer layers are capable of being sealed to each other through fusing together after heating and compressing so as to obtain tight and permanently durable sealing joints. In this manner the packing material after forming, e.g. through folding or in some other manner, can be made to retain its given shape in that the outer layers of the packing material are placed against, and sealed to, each other. A form of package manufacture which has found much application consists in that a web of packing material is formed to a tube by combining the longitudinal edges of the web in an overlap joint and sealing the web edges to each other. The tube formed can then be filled with contents, e.g. a liquid, whereafter the tube filled with liquid can be divided up to individual containers through transverse sealing of the tube along narrow zone placed at equal distances from each other. The sealed tube portions may be given before, or in connection with, the transverse sealing a lasting, e.g. parallelepipedic, shape through folding of the material, whereupon the closed tube portions are separated from the rest of the tube and form individually finished consumer packages.
It has been found that the sealing of packing material is facilitated considerably if the packing material comprises a layer of metal foil, preferably aluminium foil, at least in the regions of the sealing joints. The reason wherefore the presence of such a metal foil layer facilitates the sealing procedure is that it is relatively easy to induce electric currents into the aluminium foil layer of the packing material with the help of a so-called inductor or coil which in principle consists of ane lectric conductor brought into the desired shape which forms one or more coil windings and which is connected to a current source which feeds high-frequency alternating current to the coil. (As a rule frequencies between 2 MHz and 100 kHz are chosen). When a high-frequency current is passed through the coil or the inductor a high-frequency magnetic field is generated around the same, and if the coil is placed close to a material containing a metal layer induction currents are induced in this metal layer which give rise to a genertion of heat in the parts to which the current is conducted. This heat generated in the metal foil layer is readily passed over to adjoining layers of thermoplastic material of, e.g. polythene, which thereby are made to soften or melt, and if the material layers wherein the heat has been generated are compressed against another similar material layer with a thermoplastic coating, the thermoplastic layers in the region of heating will fuse together to form a tight and durable joint. In order to concentrate the magnetic field and to obtain narrow heating zones it is appropriate to arrange the inductor as near as possible to the metal foil layer, that is to say the inductor ought to be pressed against the material which is to be sealed, and the magnetic filed can be concentrated further with the help of an insert or a core of ferrite material which is arranged between the coil windings in the inductor. On sealing a tube it is desired in most cases that each seal should comprise two narrow sealing zones which are parallel and arranged relatively near to each other, the sealed tube being separated in the unsealed region between the said narrow sealing zones.
It is also known that in order to obtain a better sealing joint the inductor can be designed in a special manner which in principle consists in that a part of the surface of the inductor facing towards the packing material is provided along a part of its surface with a projecting ridge which means that the packing material along a part of the sealing zone is subjected to a substantially higher sealing pressure, since the packing material within this zone is compressed through the said projecting portions of the inductor. However, this sealing pressure can become so great that a "cut-effect" is obtained, that is to say that the inside plastic layer along the edge of the projecting ridge is compressed to such an extent that the plastics is wholly forced out and in practice forms a "cut" in the continuous inner plastic coating. As a rule this is of no importance, since this cut-effect occurs within the selaing joint and thus normally does not bring about any risk of leakage. It has been found, however, that in certain cases, and especially if modern high-frequency generators are used where the frequency conversion is done with the help of thyristors and transistors, the feed frequency has to be kept lower than is the case with generators where the frequency conversion is done with the help of electrn tubes. An effect which arises if a feed frequency is used which is substantially lower than that used previously, e.g. 200 kHz instead of 1.0 MHz as used normally, has turned out to be that the magnetic field around the inductor, and consequently also the electric circuit obtained in the metal foil layer in the edge portions of the packing material, that is to say along edge zones where the metal foil coating edges end, will be more rounded, which means that the sealing join along its end edges will be slightly bent. This in turn means that the said cut-effect which is obtained along the inserted portion of the inductor will produce a weakening of the inner plastic layer which owing to thr rounding of the sealing joint will have the result that the effective width of the sealing joint in the edge zones will be appreciably reduced and, in the worst case, the sealing may be absent altogether in the edge zones owing to the sealing joint curving in "behind" the cut-line or weakening line which is brought about with the help of the said projecting ridge of the inductor.
This phenomenon can be remedied, however, in accordance with the present invention if the sealing arrangement is designed in the manner characterizing the invention.